The present invention relates to servicing a well bore and, in certain embodiments, to the introduction of compositions into a well bore to reduce the loss of fluid into the formation.
A natural resource such as oil or gas residing in a subterranean formation can be recovered by drilling a well bore into the formation. A well bore is typically drilled while circulating a drilling fluid through the well bore. Among other things, the circulating drilling fluid may lubricate the drill bit, carry drill cuttings to the surface, and balance the formation pressure exerted on the well bore. One problem associated with drilling may be the undesirable loss of drilling fluid to the formation. Such lost fluids typically may go into, for example, fractures induced by excessive mud pressures, into pre-existing open fractures, or into large openings with structural strength in the formation. This problem may be referred to as “lost circulation,” and the sections of the formation into which the drilling fluid may be lost may be referred to as “lost circulation zones.” The loss of drilling fluid into the formation is undesirable, inter alia, because of the expense associated with the drilling fluid lost into the formation, loss of time, additional casing strings and, in extreme conditions, well abandonment. In addition to drilling fluids, problems with lost circulation may also be encountered with other fluids, for example, spacer fluids, completion fluids (e.g., completion brines), fracturing fluids, and cement compositions that may be introduced into a well bore.
One method that has been developed to control lost circulation involves the placement of lost circulation materials into the lost circulation zone. Conventional lost circulation materials may include fibrous, lamellated or granular materials. The lost circulation materials may be placed into the formation, inter alia, as part of a drilling fluid or as a separate lost circulation pill in an attempt to control and/or prevent lost circulation. For a number of reasons, use of lost circulation materials may not provide a desirable level of lost circulation control in all circumstances.
Another method that has been developed to control lost circulation involves the placement of a settable composition into the well bore to seal the lost circulation zone. To be effective, the settable composition should ideally maintain a low viscosity while under shear, but, when allowed to remain static, the composition should develop gel strength quickly with the ability to thin and flow when shear is re-applied. Rapid development of compressive strength is also desired after placement into the lost circulation zone. Conventional low-density cement compositions can be used, but typically do not exhibit the properties to successfully seal the zone. Faster setting compositions that can be used include, for example, mixtures of clay and aqueous rubber latex or hydratable polymers, which can become semi-solid upon contact with the drilling fluid, sealing the lost circulation zone. Cement can be added to these systems where additional strength is desired. Drawbacks to these faster setting compositions include lack of bonding to the subterranean formation, lack of ability to thin when shear is re-applied and dependency upon mixing of two streams, which can be very difficult to control.